1. Field of the Invention
The present invention relates to orthogonal drives, and in particular, to drives employing a mechanism to produce a speed difference.
2. Description of Related Art
In various industries, it is desirable to derive power from a main shaft and distribute that rotating power at right angles to the main shaft. Power is commonly distributed in that fashion in the printing industries, where a number of web handling stations are arranged in a line to perform such tasks as printing, die cutting, folding, stacking etc. In these situations the various web handling stations must remain synchronous to keep registration on the web.
Keeping registration between stations will often require adjustments to the speed of individual web handling stations. Some stations may need to run slightly faster to account for stretching of the web. Also, temporary speed adjustments may be necessary to bring the stations back into registration after being perturbed.
Known orthogonal drives employ a gear box containing of pair of bevel gears. A main shaft that protrudes through opposite sides of the gear box drives a first bevel gear. A second bevel gear is driven by the first to deliver an output at right angles to the main shaft. (As used herein, the term "bevel gear" is defined to include both straight bevel gears and spiral bevel gears.)
Adjustments to the output speed of a main drive shaft can be achieved with a class of mechanisms known as differential drives. One known differential drive is the planetary gear. Another known differential drive is the harmonic drive. The harmonic drive has a rigid drum with internal splines surrounding a flexible cup with external splines. The flexible cup is too small to engage all the splines of the rigid drum, but can be distorted into an elliptical or other shape to engage the rigid drum at two opposite poles along a major axes. A wave generator is rotatably mounted inside the flexible cup to distort it along an axis that can be rotated by the wave generator. If the wave generator rotates relative to the flexible cup, the meshing points orbit within the rigid drum. Since the flexible cup will have fewer splines than the rigid drum, the flexible cup will rotate relative to the rigid drum in the opposite direction as the wave generator, but at speed that can be varied by the wave generator.
In U.S. Pat. No. 5,570,633 a number of printing stations are each provided with an orthogonal gear box. This right angle drive provides an output shaft at right angles to a main shaft. This output is applied through a gear train to an input of a harmonic drive, which is spaced from the orthogonal gear box. The output of the harmonic drive is used to power the associated printing station. This output speed can be adjusted by a stepper motor that rotates another input of the harmonic drive. A disadvantage with this mechanism is the relative complexity of the drive trains and the substantial separation between the orthogonal drive and the harmonic drive. This complexity increases the required number of components, which adversely affects reliability, efficiency, spatial compactness, etc.
For other assemblies using a common shaft to power a number of printing stations see U.S. Pat. Nos. 1,138,782 and 2,115,975. These references do not employ a differential drive for speed adjustment. For a combination of bevel gears interconnected by a common shaft to drive a developing unit and a photoconductive drum, see U.S. Pat. No. 5,319,418. A common shaft driving a number of bevel gears to operate a louver sign is shown in U.S. Pat. Nos. 5,161,421 and 5,511,330. See also U.S. Pat. Nos. 2,916,948; 3,473,410; 4,047,451; 4,090,413; 5,509,866; and 5,634,374.
Accordingly, there is a need for an orthogonal drive with a capability of adjusting its output speed through a mechanism that is relatively compact, reliable, and efficient.